The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Clutches find use in the drive train of motor vehicles for the purpose of controlling the locking ratio of differentials or for adjusting the torque distribution between two driven axles. This requires a very accurate and reproducible adjustment of the torque that is to be transmitted by the clutch. The clutch must be capable of being disengaged very quickly (brakes with ABS) or it must disengage itself (in case of control failure); it must be also able to be engaged quickly and to be held in one position. In addition, the actuator should be able to apply the considerable force needed to press the disks together while weighing as little as possible and occupying a small mounting space.
A clutch of this kind is described in WO 01/59331 A1. In this clutch, the rotatable rings carry levers, the ends of which ride on a rotatable control disk driven by a geared electric motor at a high gear ratio. Because of the plays and frictional losses in the geared motor that can lead to self-locking which would render fast disengagement impossible and, furthermore, because of the high cost, high weight, and difficult installation, this approach is unsatisfactory.
Moreover, it is known from U.S. Pat. No. 4,976,347 to provide each of the two rotatable rings with a toothed gear which meshes with a toothed gear on the motor shaft. With a slight difference in tooth number, it is possible to achieve a high gear ratio without the clutch having its own speed reduction gear. In this manner, however, considerable frictional losses arise in the gear tooth system which can lead to self-locking. Such locking must be prevented, however, if in the event of control failure the clutch is to release itself. In the event of fast actuation, shocks occur which raise the load on and wear of the actuator as a result of the peculiarities of the gear tooth system (i.e., backlash and resistance to rotation of the system). The friction not only causes an energy loss, but it also makes it more difficult to adjust the torque to be transmitted by measuring and controlling the motor current. The design of the gear tooth system is difficult to reconcile with the installation requirements.
Hence, it is desirable to provide an actuator which operates as accurately and friction-free as possible, and which at a lower cost and reduced weight can readily be accommodated in a housing.